In keeping with a recent trend of energy saving, attention has been given to environmental energy that does not depend on fossil fuel or the like and exists ordinarily. Photovoltaic energy, wind energy and the like have been widely known as environmental energy. In addition, vibration energy existing around ordinarily can be mentioned as environmental energy having an energy density which is not inferior to that of the foregoing energy. In order to supply, to a load circuit, electric power generated from this vibration energy by an electric power generation device, heretofore, a circuit for converting a voltage so as to be suitable for the load circuit has been under study with regard to improvement in its conversion efficiency.
For example, there has been disclosed a technique of preventing reduction in voltage conversion efficiency in consideration of a fact that it becomes difficult to realize a desired voltage drop because of output impedance of a vibration electric power generation device (refer to Patent Document 1). According to this technique, switching circuits are provided for switching, between series connection and parallel connection, states of connection of a plurality of capacitors to a power source circuit and a load circuit. Thus, the number of capacitors to be connected by the switching circuits is controlled on the basis of an input voltage from the power source circuit or a frequency of the input voltage. Moreover, Patent Document 2 discloses a technique for efficiently generating electric power from small-amplitude vibrations. According to this technique, the ON/OFF control of a switch for controlling an electric power output from a vibration electric power generation device is carried out on the basis of a cycle of vibrations, so that an electric power output cycle is varied.